The invention relates to a display device comprising an electro-optical display medium between two supporting plates, at least one supporting plate being provided with a picture electrode and a metal pattern which is light-reflecting at the side facing the supporting plate.
The invention also relates to the manufacture of a metal pattern on a supporting plate; such a supporting plate may comprise switching elements such as diodes, transistors etc. and also electrodes which may function as picture electrodes in a display device.
A display device of this type is suitable for displaying video information and alpha-numerical information by means of electro-optical display media such as liquid crystalline materials.
A display device of the type described in the opening paragraph is known from JP-A 3-256025. This document describes a liquid crystal display device in which a metal pattern constituting a black matrix is provided on one of the supporting plates. The metal pattern is formed from a layer of highly reflecting material such as aluminium. Incident light is reflected at the location of the black matrix so that it cannot be incident on the switching elements arranged opposite this black matrix, which elements are usually light-sensitive. The metal pattern is coated with a layer of chromium oxide to prevent unwanted reflections in the layer of liquid crystalline material so that a better contrast is obtained.
A drawback of such a structure of the black matrix pattern of two layers is that this pattern may become relatively thick (approximately 0.5 .mu.m), which leads to poor step coverages of electrodes which may be provided at a later stage. The provision of colour filters, orienting layers or protective layers may also give rise to problems in the case of a large thickness.
Moreover, the layer of chromium oxide provided on the metal pattern has a reflective power of approximately 10% so that not all consequences of internal reflections are eliminated. Notably in the case of use in projection display devices such a quantity of light may be incident on the switching elements that, for example leakage currents are introduced. Also the contrast may be detrimentally influenced by internal reflections.
In addition, problems occur in the manufacture of such double layers, notably when chromium oxide is provided by means of reactive sputtering. Without a very accurate process control, areas with a colour different from black may be produced in the layer of chromium oxide (discolouration).